-Noot symbolism-
Due to being the guardian of the Tree of Feelings' negative half and being made of oure negativity, Nightmare is often represented by rot and decay or wicked and cruel things, hence the few flowers and their darker colors. He has a hard time with happy or positive emotions, and is portrayed or seen as a silent depressoid... I.... that's blatant disrespect but blame Clindsdale-

-Scream symbolism-
Dream guarded the positive half of the Tree of Feelings, and is represented by all things good and pure. She can heal and purify waters or toxins, ripen fruit and make flowers bloom and, like her brother, draws power from her respective emotions spectrum. As her brother was hated and ashamed by their homeland's townsfolk, she was loved and her arrival regarded with flowers and oil or wine.

They also represent seasons: Dream with summer for it's tender heat and bright sun and spring for the living forms. Nightmare represents fall in where the plants die and animals begin slumber, and winter for its merciless and biting cold and winds, and for the death it so often brings.